S. mutans, among other bacteria, produce acids from sugar that is consumed, and these dissolve the hard enamel coating on teeth.

The bacteria are usually transmitted from mothers, and the age of highest risk is at 19-31 months.

Secondhand smoke detrimental to oral health

Secondhand smoke may directly affect teeth and microorganisms in a number of ways, including inflammation of the oral membrane, damage to the salivary gland function and a decrease in serum vitamin C levels, as well as immune dysfunction.

Children exposed to passive smoking also have lower salivary IgA levels and higher levels of sialic acid with higher activity. Sialic acid enhances the agglutination of S. mutans, leading to the formation of dental plaque and caries.

This would suggest that reducing secondhand smoke among children could help prevent caries, although it has not been proven.

A team of researchers based in Japan set out to investigate smoking during pregnancy and exposure to household smoke in infants at 4 months of age as risk factors for caries in deciduous teeth.

They wanted to know whether maternal smoking during pregnancy and exposure of infants to tobacco smoke at the age of 4 months would increase the risk of caries in deciduous teeth.

The team analyzed data for 76,920 children born between 2004 and 2010, who attended routine health checkups at 0, 4, 9 and 18 months, and at 3 years of age at health care centers in Kobe City, Japan.

Mothers completed questionnaires to provide information about secondhand smoke exposure from pregnancy to 3 years of age and other lifestyle factors, such as dietary habits and oral care.

Incidence of caries in deciduous teeth was defined as at least one decayed, missing or filled tooth assessed by qualified dentists.

High risk of caries in children of parents who smoke

Prevalence of household smoking among children included in the study was 55.3%, and 6.8% showed evidence of tobacco exposure. A total of 12,729 cases of dental caries were identified, mostly decayed teeth.

Compared with having no smokers in the family, exposure to tobacco smoke at 4 months of age was associated with an approximately twofold increase in the risk of caries. The effect of maternal smoking during pregnancy was not statistically significant.

Limitations include the fact that this was an observational study, so conclusions about cause and effect cannot be drawn; results may also have been influenced by other unmeasured factors.

Despite the limitations, the researchers conclude:

"Exposure to secondhand smoke at 4 months of age, which is experienced by half of all children of that age in Kobe City, Japan, is associated with an increased risk of caries in deciduous teeth. Although these findings cannot establish causality, they support extending public health and clinical interventions to reduce secondhand smoke"